1. Field of the Invention
This invention relates, generally, to aircrafts. More specifically, it relates to aircrafts convertible between lighter-than-air and heavy-than-air configurations.
2. Brief Description of the Prior Art
Lighter-Than-Air (LTA) aircrafts have some characteristics not shared with Heavier-Than-Air (HTA) aircrafts in that they can maintain altitude without moving in a medium and can do so as long as buoyancy is maintained. LTA aircrafts use low-density gas, such as helium or hydrogen to float in higher density air. These aircrafts usually employ one or more gasbags filled with low-density gas to create a buoyancy force that offsets the weight of the aircraft. The downside of LTA aircrafts is their large size, which is accompanied by large drag characteristics, preventing them from traveling at higher speeds. The current speed record for an LTA aircraft is 112 Km/hr (69.6 mph).
HTA aircrafts use Newton's third law and Bernoulli's principle to achieve flight. These aircrafts are generally fixed wing or rotor wing aircraft. In either case, part or parts of the structure (e.g., wing, rotors, propellers, fuselage, and control surfaces) have a characteristic shape called an airfoil. Airfoils are generally asymmetric in cross-section with the upper surface having a greater length than the lower surface. This causes air moving across the upper surface to travel faster than the air traveling across the lower surface causing a pressure decrease on the upper surface resulting in lift.
Lift can also be achieved/altered by altering the angle of attack (AoA) of an airfoil relative to the oncoming airflow. Increased AoA causes mass deflection resulting in lift (Newton's third law). Generally, increasing AoA increases lift until the angle reaches a point at which the airflow separates from the surface of the airfoil causing aerodynamic stall.
Regardless of means for creating lift, an HTA requires a wing-like structure moving through a fluid. Movement requires a power source and no power source can last indefinitely. Therefore, the HTA aircrafts can only maintain flight for limited periods of time. Even powerless gliders have duration limits as they trade airspeed for altitude gained from thermal lift. The limitation in flight time of an HTA aircraft, however, is compensated by low drag characteristics and thus, high-speed flight.
The clear tradeoff between LTA and HTA aircrafts is speed verse indefinite flight. An ideal aircraft would have the ability of an LTA to hover, or station-keep, for extended periods for observation or surveillance roles and also the ability of an HTA to operate at high-speeds. This can theoretically be achieved through an aircraft convertible between an LTA and an HTA configuration. Currently, there exist hybrid convertible aircrafts, but none that provide a unique combination of attributes of both a fixed wing aircraft and a LTA craft allowing for indefinite mission durations, low energy station keeping, and the ability to dash at relatively high velocities.
U.S. Pat. No. 5,005,783 to James D. Taylor teaches a variable geometry airship capable of converting between a LTA and HTA airship. However, the airship is operationally complex and does not extend the operating range sufficiently to be practical as shown in Table 1. Along with multiple other pitfalls, the shape and design of this airship prevents the airship from transforming into both a symmetric neutral lift configuration and a negative lift configuration, thereby reducing the effectiveness of the airship.
U.S. Pat. No. 4,102,519 to Edward L. Crosby, Jr. teaches a variable lift inflatable airfoil. However, this invention lacks internal moveable structures, which prevent the airfoil from achieving multiple configurations. Additionally, the airfoil lacks a propulsion system and/or control surfaces.
Accordingly, what is needed is an improved variable geometry aircraft having a simple, moveable internal structure to easily convert the aircraft between an LTA configuration and an HTA configuration. However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the field of this invention how the shortcomings of the prior art could be overcome.
All referenced publications are incorporated herein by reference in their entirety. Furthermore, where a definition or use of a term in a reference, which is incorporated by reference herein, is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
While certain aspects of conventional technologies have been discussed to facilitate disclosure of the invention, Applicant in no way disclaims these technical aspects, and it is contemplated that the claimed invention may encompass one or more of the conventional technical aspects discussed herein.
The present invention may address one or more of the problems and deficiencies of the prior art discussed above. However, it is contemplated that the invention may prove useful in addressing other problems and deficiencies in a number of technical areas. Therefore, the claimed invention should not necessarily be construed as limited to addressing any of the particular problems or deficiencies discussed herein.
In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge, or otherwise constitutes prior art under the applicable statutory provisions; or is known to be relevant to an attempt to solve any problem with which this specification is concerned.